“Proteins with environmental stress resistance” refer to proteins that physically, chemically and biologically show stability against external environmental factors such as heat, pH, metal ions, organic solvents, etc. Typically among such proteins, there are heat-stable proteins which are stable even at the boiling temperature of water. One group of heat-stable proteins is represented by proteins derived from hyperthermophilic organisms [Jaenicke R. and Bohm G., Curr. Opin. Struct. Bio., 8, 738–748 (1998); Ress D. C. and Adams M. W. W. Structure, 3, 251–254 (1995); and Adams M. W. W., Ann. Rev. Microbiol. 47, 627–658 (1993)]. These proteins have an extremely high melting temperature (hereinafter referred to as “Tm”), relative to their mesophilic counterparts (near or above the boiling point of water). However, when the temperature is increased above the Tm, most hyperthermophilic proteins also denature, leading to insoluble aggregation [Klump et al., J. Biol. Chem., 267, 22681–22685 (1992); Klump et al., Pure. Appl. Chem., 66, 485–489 (1994); Cavagnero S. et al., Biochemistry, 34, 9865–9873 (1995)].
Another group of heat-stable proteins, which has been recently recognized, is the intrinsically unstructured proteins [Plaxco, K. W. and Groβ M., Nature, 386, 657–658 (1997); Wright P. E. and Dyson H. J., J. Mol. Biol., 293, 321–331 (1999)]. The reason why the intrinsically unstructured proteins are heat-stable is because the conformation of the intrinsically unstructured proteins is not extensively changed by heat treatment. Thermodynamically, the intrinsically unstructured proteins are heat resistant proteins (hereinafter referred to as “HRPs”) rather than heat-stable proteins since their conformation almost unfolds at room temperature and is somewhat changed at high temperatures (Kim T. D. et al., Biochemistry, 39, 14839–14846 (2000)). Thus, the term “heat resistant proteins (HRPs)” is more appropriate for describing the thermal behavior of the intrinsically unstructured proteins. That is, HRPs can be defined as proteins that are not aggregated by heat treatment, such as hyperthermophilic proteins and unstructured proteins.
The thermal behavior of proteins was systematically investigated by purifying and characterizing some HRPs that are not aggregated by heat treatment from Jurkat T cells and human serum (Kim T. D. et al., Biochemistry, 39, 14839–14846 (2000)). According to studies on the heat resistance of proteins from Jurkat cell lysates and human serum, four major types of thermal behavior of HRPs were recognized, which are as follows. Group I HRPs are represented by unstructured proteins such as α-synuclein and αs-casein, which have a semi-unfolded conformation regardless of temperature. Group II HRPs, represented by human serum fetuin and albumin, are characterized by an irreversible conformational change upon heat treatment. Group III HRPs, represented by transthyretin and bovine serum fetuin, are characterized by a reversible conformational change. Group IV HRPs, conventional heat-stable proteins such as hyperthermophilic proteins, are characterized by the absence of heat induced conformational changes.
Most proteins unfold and in turn precipitate as the temperature increases, and the process is usually irreversible (Bull H. B. and Breese K., Arch. Biochem. Biophys., 156, 604–612 (1973)). The improvement of stress resistance, including the improvement of thermal stability, is one of the tasks to be solved for proteins, such as hormones, cytokines and enzymes, widely used in the medical or industrial fields. Improvement of stress-resistance, of course, renders the life span of products to be elongated, thereby leading to development of novel medical products and more stable industrial enzymes, foods or chemical products. Therefore, the present invention relating to novel stress-resistant proteins will be very useful.